As noted in U.S. Pat. No. 3,888,973 to Colwell et al, passing a liquid reactant through a bed of solid particle reactants by gravity or hydrostatic pressure is known. Also known, as exemplified by Japanese patent document No. 52-15496, is the production of ferrous nitrate by passing ferric nitrate through a tube filled with pieces of scrap iron.
As illustrated by U.S. Pat. No. 1,385,769 to Ball, iron compounds may be produced by passing upwardly through a vessel containing pieces of metallic iron, a stream of an oxidizing medium including a solution of ferrous sulfate, and air. When air is admitted under pressure, it causes violent agitation of the ferrous sulfate solution and exerts a lifting action, causing a rise of the solution within the vessel and overflow.
U.S. Pat. No. 2,581,519 to Critchley exemplifies the manufacture of a metal nitrate from nitric acid and a suitable metal such as silver, by immersing a column of bars of the metal in a cold solution of the metal nitrate, heating while displacing air from the solution with oxygen, simultaneously introducing nitric acid and oxygen, and displacing the metal nitrate solution with freshly formed nitrate solution.
As illustrated by the Dictionary of Commercial Chemicals, 3rd Ed., the production of ferric nitrate by dissolving scrap iron in concentrated nitric acid, and crystallizing, is known. However, passivation of the metallic iron reactant is a serious problem. As a result of passivation, the reaction will proceed slowly and stop before the scrap iron is completely dissolved.
Passivation would be lessened by heating. However, a substantial limitation on the usefulness of heating to lessen passivation, is that the ferric nitrate-to-ferric oxide decomposition temperature is about 100.degree. C. Ferric oxide contamination is undesirable. Furthermore, the use of heat typically translates into increased cost.
Dissolving iron powder, rather than scrap iron, in concentrated nitric acid would substantially overcome the passivation problem. However, iron powder is a prohibitively expensive starting material.
Thus, the discovery of a low cost, economical process for making ferric nitrate that overcomes the passivation problem, would constitute a significant contribution to the art. Such a process could be used for manufacturing an aqueous ferric nitrate solution containing a high concentration of soluble ferric iron. By the term "high" in this context, is meant, for purposes of this invention, 8% or more of the ferric iron. Such a process would contribute even further to the art if it provided an aqueous ferric nitrate solution high in soluble ferric iron, further characterized by less than 300-500 ppm of insoluble ferric oxide.